1. Field of the Invention
The present invention relates generally to improved methods for forming "hot end" coatings for vitreous articles such as glassware, and more particularly to new and improved methods for forming zinc oxide and magnesium oxide "hot end" coatings upon glassware from aqueous based solutions, and to articles so coated.
2. Description of the Prior Art
The desirability of forming refractory coatings on vitreous surfaces, such as glass, and particularly upon glassware such as bottles, is well known. Pristine glass is theoretically an extremely strong substance. However, when, for example, glassware is subjected to high speed packing and processing machines and normal use in which glass is caused to rub against glass, scratches, abrasions, and other physical damage rapidly degrades the surface of the glass thereby providing sites for initiation of failure throughout the glass. Also, glass is subject to attack by various atmospheric constituents, particularly moisture. Accordingly, it has been an accepted practice in the production of glassware to form a refractory, so-called "hot end" coating upon the newly formed glass, usually at or before the annealing lehr while the glassware is at an elevated temperature. After formation of the refractory "hot end" coating, it is conventional practice to thereafter apply a lubricious "cold end" coating at lower temperatures. "Cold end" coatings conventionally include, for instance, waxy polyethylene, fatty acids, beeswax, and other such lubricious compositions either alone or in combination. Neither the "hot end" coating or "cold end" coating is protective, and alone provides little protection against scratching by a similarly coated vitreous article.
The "hot end" coatings, to which the present invention pertains, display particular requirements which distinguish such "hot end" coatings from a number of other coatings previously employed upon glass for various purposes. As an example, thick coatings of tin oxide and magnesia have been applied to, for instance, electrical insulators. Other thick coatings of tin oxide have been applied to provide electrical conductivity for defrosting of glass surfaces by employing the tin oxide coating as a resistance element. Semi-transparent, colored coatings have been utilized for decorative purposes and, in some instances, for tinting of glass.
"Hot end" coatings have a number of requirements not met by many glass coatings. Typically, such coatings should be non-toxic as a coating, and also are desirably formed from compositions which are not in themselves toxic and which do not form toxic by-products. The coatings formed should not change the appearance of the glassware, i.e., should be colorless and substantially free of iridescence. Contrary to the purpose of certain other glass coatings, "hot end" coatings should have high electrical resistance to avoid galvanic action with container caps and moisture which leads to unsightly cap corrosion and/or increased torque requirements. Since glassware is formed at high rates, coatings must be conveniently applied and utilize relatively economical solvents and compounds. Further, "hot end" coatings must be resistant to normal handling and processing, and must tenaciously adhere to the glass and provide a suitable base for lubricious "cold end" coating.
Thus, it will be recognized that "hot end" coatings for glassware are a well recognized group of materials having characteristics and requirements distinct from other glass coatings.
Currently, "hot end" coatings are generally tin oxide or titanium oxide, formed from vapors of anhydrous tin or titanium tetrachloride or from aqueous solutions of such compounds. However, previously "hot end" coatings of tin, titanium or zirconium oxide have also been formed from organic compounds applied in or from organic solutions.
A number of prior art discussions exist concerning the various coatings. For instance, U.S. Pat. No. 2,165,819, issued July 11, 1939, discloses an electrical insulator of magnesium titanate suitable for formation on ceramic substances such as condensors. Magnesium titanate is disclosed as being suitable for a narrow, and quite specific coating purpose, but does not correlate with the requirements of a "hot end" coating.
U.S. Pat. No. 3,323,889, issued June 6, 1967, discloses a coating system for the purpose of the instant invention, but involves the use of titanium oxide as a "hot end" coating applied from an organic solvent solution in conjunction with relatively conventional "cold end" coatings. Organic solvents are expensive, often flammable, i.e., as is tetraisopropyl alcohol and cause objectionable pollution of the atmosphere. However, U.S. Pat. No. 3,323,889 is pertinent in setting forth many of the requirements of a "hot end" coating.
U.S. Pat. No. 3,450,574, issued June 17, 1969, is concerned with the preparation of a magnesia coating on refractory bodies. Preferable, the patent teaches the formation of a thin film of metal under reducing conditions and thereafter providing oxidizing conditions to form the oxide. Sintering techniques are also disclosed. Clearly this patent does not pertain to "hot end" coatings contemplated by the instant invention.
U.S. Pat. No. 3,516,811, issued June 23, 1970, primarily discloses apparatus for applying "hot end" coatings in laminar flow conditions, but also contains a fairly comprehensive listing of various prior art patents dealing with "hot end" and "cold end" coatings.
Typical of the hot end coatings discussed by U.S. Pat. No. 3,516,811 is that of U.S. Pat. No. 3,561,940, issued Feb. 9, 1971, which concerns the formation of tin oxide coatings by the pyrolytic composition of anhydrous stannic tetrachloride vapors in a moisture-free carrier gas. While tin oxide is a desirable and workable "hot end" coating, the by-products of the pyrolytic decomposition include hydrochloric acid (upon hydrolysis with atmospheric moisture) which presents serious problems with regard to attack of surrounding equipment and pollution of the atmosphere. Such installations may require expensive scrubbing equipment to remove the unused stannic tetrachloride and hydrochloric acid from the exhaust gases.
U.S. Pat. No. 3,694,299, issued Sept. 26, 1972, discloses a means for fusing glass sheets together utilizing an organo-metallic film forming solution in which the metallic constituent may include magnesium. However, the solution utilizes volatile and combustible solvents as well as resins which would be entirely inappropriate for use in a "hot end" coating composition for the reasons discussed above.
U.S. Pat. No. 3,711,322, issued Jan. 16, 1973, discloses organic solvent based compositions utilizing two metal compounds, one of which may be a magnesium compound such as magnesium acetate, to form semi-transparent, colored metal oxide coatings. For numerous reasons including transparency, color, organic solvents, etc., such coatings would not be useful as "hot end" coatings.
U.S. Pat. No. 3,847,583, issued Nov. 12, 1974, similarly discloses the use of two metal compounds dissolved in an organic solvent to form coatings. Titanium oxide and, it is believed, tin oxide, are discussed, and specific mention is made that alkali and alkali earth oxides are not operable to form pyrolytically induced oxide coatings.
U.S. Pat. No. 3,926,103, issued Dec. 16, 1975, discloses a recovery system for spray solutions of tin compounds.
Finally, U.S. Pat. No. 3,984,591, issued Oct. 5, 1976, discloses the formation of metallic oxide coatings from metallic salts utilizing, as a critical feature of the invention, aprotic solvents of specified dipolar moments which would be inappropriate for "hot end" coatings. Mention is made of spraying aqueous solutions, but such coatings are characterized as being readily removed in the form of dust by passing a finger over the coating. "Protective" coatings are taught to be tin oxide, zirconium oxide and titanium oxide.
Thus, on the basis of the above prior art, the only workable "hot end" coatings are those involving pyrolytic decomposition from objectionable organic solvents or from halides. In both cases, objectionable by-products in the form of exhaust fumes are presented. The better, more economical coatings involve pyrolytic decomposition of stannic tetrachloride with the resulting formation of hydrochloric acid.